Not Applicable.
The present invention is generally related to the field of high-speed data communications, and, more particularly, to a system and method for shortening the impulse response of a data communications channel.
In data communications using discrete multi-tone (DMT) modulation techniques, equalization and other filtering in the time domain are often employed to reduce or eliminate data signal distortion, intersymbol interference, inter-channel interference, and noise introduced by a data communications channel. Since DMT receiving techniques sample and digitize an analog data signal received from a communications channel, an anti-aliasing filter is applied in order to prevent spectral aliasing in the sampled signal.
Anti-aliasing filters can be implemented either entirely in the analog domain, or partially in the analog domain and partially in the digital domain, filtering an over-sampled received signal. Either way, the desired end result is that there be no spectral aliasing when the fast Fourier transform (FFT) is applied to the received signal samples. Conventional DMT techniques sample the received signal at the Nyquist rate, i.e., twice the maximum signal bandwidth, so that each bin at the FFT output corresponds to a data-encoding channel.
However, such anti-aliasing filters, when set to the Nyquist cutoff frequency, introduce either a loss of signal strength, or some spectral aliasing, or both, near the Nyquist frequency, depending upon the exact frequency response of the anti-aliasing filter. The consequence of this loss of signal strength or spectral aliasing is reduced modem performance. Also, such filters feature a sharp cutoff, requiring either an extensive digital filter with a relatively large number of taps, or an extensive analog filter which is employed before digitizing the analog data signal, which increases the cost of the DMT modem. Additionally, such sharp filters may create a substantial time dispersion that further increases the burden of time-domain equalization.
The present invention entails a system for signal conditioning and equalization in the time domain, preferably in a discrete multi-tone (DMT) modem. The system includes an analog front end, comprising an anti-aliasing filter and an analog-to-digital (A/D) converter operating at a higher-than-Nyquist rate, and a processor operating according to operating logic stored in its memory. The operating logic includes logic executing the function of a low pass filter having a predetermined cutoff frequency configured to process a digitized data signal having a first sample rate f0 received from the A/D converter, producing a filtered data signal at a second sample rate f1. The operating logic further includes logic which executes the function of an adaptive time domain equalizer, the time domain equalizer being configured to process the filtered signal to shorten the impulse response of the channel while at the same time, the time domain equalizer down-samples the filtered signal from the second sample rate f1, to a third sample rate f2 for further processing in the frequency domain.
The present invention provides distinct advantages in that the oversampling feature allows a folding of the power spectral density of the received data signal in a way that enhances the received signal power. In addition, there is no need for an anti-aliasing filter with a sharp cut-off characteristic, that is expensive, difficult to implement, and increases a burden of the equalizer. Rather, a mild low pass filter is employed, requiring fewer taps and, consequently, less computing power and memory.
Alternatively, the mild low-pass filtering function can be implemented in the analog domain, preferably served by the anti-aliasing filter for oversampling. It should be noted that the anti-aliasing filter for an over-sampling equalizer is easier to implement since it does not require a sharp cutoff and its cut-off frequency need not be accurate.
In accordance with another aspect of the present invention, a method is provided for shortening an impulse response of a data communications channel in the time domain in a discrete multi-tone (DMT) modem. The method preferably is applied to a digitized data signal received from a first DMT modem, the digitized data signal having a first sample rate f0. The method comprises the step of digitally attenuating the digitized data signal above a predetermined cutoff frequency, producing a filtered signal at a second sample rate f1. Next the step of processing the filtered signal is performed in a time domain equalizer, in which the filtered signal is down-sampled from the second sample rate f1, to a third sample rate f2, wherein f0 is an integer multiple of f1, and f1, is an integer multiple of f2.
Other features and advantages of the present invention will become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional features and advantages be included herein within the scope of the present invention, as defined by the claims.